High-efficiency light absorbing and emitting nanostructures are critically needed for a large range of opto-electronic devices and systems applications, ranging from solar cells and simple detectors to advanced light emitter-based applications, including those based on multiphoton light absorption. As a case in point, two-photon absorption-induced fluorescence (TPAF) has been demonstrated as a powerful nonlinear optical phenomenon for several bio-imaging applications, particularly for deep-tissue imaging and for photodynamic therapy. In photodynamic therapy, the photon generated by two-photon upconversion is used to generate cytotoxic reactive oxygen species (ROS) in cancer tissue. Focusing intense near-infrared radiation (NIR) in cancer tissue that is relatively transparent to the NIR, whose wavelength is in the tissue optical transparency window of 600-1300 nm, can result in deep tissue penetration followed by selective destruction of malignant cells via efficient TPAF-induced ROS generation. Additional targeting of specific tissue can also be achieved by functionalizing the TPAF nanoparticles with biomolecules to cause increased accumulation in the target tissue, both for photodynamic therapy and for imaging applications.
Modeling of nanostructures can include analysis using the permittivities of materials assumed to be part of the nanostructures being modeled. In such modeling, the bulk dielectric constants of the materials may be typically used. However, there are some recent experimental efforts at measuring the permittivity of ultra thin layers of noble metals. In two articles, the measured permittivity for gold films from 2-10 nm thick on a silica substrate was reported. In an article, the use of picometrology was reported with measurements at 532 nm and 488 nm wavelengths, which are not useful for TPAF in most tissues. The use of ellipsometry to determine the complex relative dielectric constant for wavelengths from 280 nm to 1.7 μm has been reported. For 800 nm, the results of measurements of the real relative dielectric constant in the range (−20, 17) and imaginary relative dielectric constant in the range (2, 30) for gold thicknesses from 3-10 nm have been reported. See, for example, M. Hovel, B. Gompf, and M. Dressel, “Dielectric properties of ultrathin metal films around the percolation threshold,” Phys. Rev. B 81, 035402 (2010), and X. Wang, K. Chen, M. Zhao, and D. D. Nolte, “Refractive index and dielectric constant transition of ultra-thin gold from cluster to film,” Opt. Express 18, 2485924867 (2010).
There has been a long-standing need for high-brightness, nonphotobleaching, and nontoxic TPAF fluorophores for numerous medical research and clinical applications. During the last decade, semiconductor quantum dots (QDs) have attracted significant attention as TPAF nanoparticle labels due to their significant advantages over other fluorophores, which include: (a) broad absorption spectra and readily tunable emission options (b) high quantum yields, (c) relatively high photochemical stability, and (d) relatively large two-photon absorption cross sections. Unfortunately, QDs frequently contain toxic elements (such as cadmium), which limits their use for in vivo clinical applications.
Studies of related subject matter have been reported in a number of articles. These articles include:
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All of the references listed above are incorporated herein by reference in their entirety.